School to home system



5 Sheets-Sheet 1 Filed Aug. 10, 1948 IN/ENTOR.

N MW W miv E d, Ay

TTORNEY' Nov. 7, 195o Filed Aug. 10, 1948 A. c. BERNSTEIN 2,529,069

SCHOOL TO HOME SYSTEM 3 Sheets-Sheet 2 -lllI INI/ENTOR.

' ATTORNEYS Nov. 7, 1950 A. c. BERNSTEIN scHooL To HOME SYSTEM Filed Aug. 1o, 194s s sheets-sheet s kmq Patented Nov. 7, 1950 UNITED STATES PATENT OFFICE 18 Claims.

rEhe present invention relates in general to the art of electrical signalling and more particularly to a novel intercommunication system enabling a plurality of persons to converse conveniently over conventional-type telephone lines.

rEhe broad teachings of the present invention, asv will be seen from the discosure below, are widely applicable to systems for interand intraoi'lice communication, lire alarm signalling, and the like. However, for the purpose of simplifying this discussion, detailed consideration will be coniined to a representative installation which currently is solving a recognized complex educational problem.

Speciiically, the problem is that of providing suitable means within iinancial reach for improving the educational facilities available to physically handicapped and bedridden children unable to attend school and pursue a normal scholastic course'. Although heretofore numerous methods have been utilized and proposed for bringing education to such children, all such methods have had serious practical limitations.

It has been long recognized that self-instruction, or instruction with the aid of a part-time tutor, is diiiicult and uninteresting and that considerable therapeutic value could be attained by the association of the bedridden child with the classroom activities of other children of the same age. Another suggested procedure is that of recording entire classroom proceedings which the handicapped child would attend if normal conditions prevailed, and reproduce these recordings in the childs home at a subsequent time.

rlhis procedure has limited application due to.

the comparatively high cost of recording lengthy class-room discussions, and suffers the further disadvantage of limiting the childs activity to listening while barring participation in any manner.

It is possible to reduce expense somewhat by direct pick-up of the classroom proceedings by microphone and transmission by radio or leased telephone line to the home of the child. Al-

though such procedure gives the child the advantage of hearing the proceedings while they are current, it still precludes participation.

The present invention contemplates and has as a primary object the provision of an electrical inter-communication system, particularly adapted for use in connection with leased telephone lines, for the exchange of information between a number of separated points, as for example, a lclassroomand the bedside of a physically handicapped child. As will be more fully disclosed hereinbelow, this novel inter-communication system permits conversation and lecture normally occurring in a classroom to be transmitted to a remote point for reproduction. By means at the control of the child, the reversal of signalling is ei'lected so that the child may interrupt classroom proceedings to ask questions or to answer in recitation as if present in the actual classroom.

The advantages of an inter-communication system of this type from a psychological and educational viewpoint are readily apparent. The child, in effect, engages in substantially all features of school activity although not actually present. By virtue of the completeness of the educational program possible and by placing the child in an active association with other children, remarkable scholastic progress has been recorded.

To accomplish signal transmission over a leased telephone line, it is essential that the signal applied thereto is of suiiiciently low level as to preclude interference with other telephone conversations iiowing on adjacent lines with which electrical coupling is possible. That is to say, the phenomenon known as cross-talk must be precluded prior to acceptance of the system by the local telephone operating exchange. As a consequence, electrical energy developed from the sound pick-up must be of a general level suitable for application to the telephone line. At the remote end of the telephone line, that is, at the childs home, the received signals must be amplied to a level suitable for loudspeaker reproduction. In accomplishing reverse signal flow when the child speaks to the class, it is essential that signal level limiting be again provided so that suitable transmission over the telephone line may be effected.

It is therefore an object of the present invention to provide an audio inter-communication system for use in conjunction with a telephone line operating at an established, comparatively low signal level while permitting two-way reproduction at comparatively high signal level.

In normal operation, the inter-communication system for use between a classroom and a student at home is adjusted so that the classroom proceedings will be picked up and transmitted directly to the remote point for reproduction. When called upon for recitation, or when desiring to interpose a question into the proceedings, the student at home, by operation of simple switching apparatus, reverses the direction of amplification at both points and the direction of audio energy flow. For design simplicity and 3 furthermore, for the purpose oi permitting operation of a single school unit in congunction with a plurality or" leased telephone lines the amplilier reversal, although actuated by the student at home, is eiected by means oi a relay at the school. The novel circuit design employed in the present inter-communication system permits utilization of a single two wire telephone line between class and student for direct and reverse audio current and for carrying the switching signals.

It is accordingly a further object of the present invention to provide an inter-communication circuit which permits two-way audio transmission and also the actuation of ampliers at op- 'posed ends of the system for reverse operation over a single telephone line with all manual switching operations being confined to one end of said telephone line.

A still further object of the present invention is to provide an electrical inter-communication system for joining a classroom with one or more handicapped students at remote points,

which permits any student to call into the classroom While all others on the system hear this call.

Still another object 01' the present invention is to provide an inter-communication system for operation'over a telephone line wherein switching is accomplished by the application of a direct current to the telephone line in addition to the audio signal iiowing therein. Y

Another object of the present invention is to provide an inter-communication system suitable for operation over telephone lines so that audio energy at one point may be transmitted successfully to a plurality of remote points while permitting persons at each of these remote points to communicate with all persons connected to the system including the point of origin.

These and other objects of the present invention will now become apparent upon consideration of the following detailed specication when taken Vin connection with the accompanying drawings in which:

Figure l is a general schematic diagram of a possible arrangement of the inter-communication system of this invention adapted for the transmission of information from a, school to various remoteY points;

Figure 2 is a schematic circuit diagram of an embodiment of the electrical apparatus contained at the school end of the transmission system; and l v Figure 3 is a schematic circuit diagram of the electrical apparatus employed at the remote end of the inter-communication system.

With reference now to the drawings and more particularly to Figure 1 thereof, there is illustrated an inter-communication system incorporating the principles of the present invention and speciiically adapted to suit the needs of the education of physically handicapped children conned to home or bed at points remote from the school. Broadly, this inter-communication system permits the two-way transfer of audio energy from one of the electrical transducers such as II and I2, located in one or more classrooms from which instruction is desired, to corresponding transducers such as I3, I4 and l5 at the homes of students at remote points. Corresponding to transducers Il and I2 at the school, there is an electrical amplication school unit i6 which serves to interchange energy suitably with the transducer element in use.

Transducer elements such as II and I2 are located in various classrooms. Any one of these transducers may be connected to the school unit I6 to permit proceedings of arparticular class to be connected with the remote point for the interchange of information. The output ofV the school unit I6 is, at terminal 22, connected to a leased telephone line extending to the central exchange 23 of the area served.

A leased line is a telephone line which is obtained for the exclusive use of the subscriber from the area telephone operating company for a specific purpose. It will be understood that one requirement of a leased telephone line operation is that the signal level applied thereto be no higher than a predetermined maximum established by the operating company as the level above which cross-talk becomes a particularly deleterious factor. In the discussion that follows on circuit design considerations, it will be assumed that maximum operating level permissible on the telephone line is known.

Radiating from the central telephone exchange 23 are a plurality of leased lines such as 24, 25 and 23, one of which is directed to each of the homes of theV physically handicapped students utilizing the system illustrated or to a party line reaching the homes of one or more of the handicapped children. The electrical apparatus required at each home is contained in what herein is termed a home unit. Thus, home units 3|, 32 and 33 are connected to leased lines 24, 25 and 25, respectively, and are adapted for the interchange of energy with electro-mechanical transducers I3, I4 and I5, respectively. Considering the system as a whole, then'two-way sound transmission is possible between electro-mechanical transducer II and each of electro-mechanical transducers I3, I4 and l5 through central eX- change 23 and over the corresponding leased lines to the home units.

As will become apparent from the detailed information to be provided below, in normal operation transducer I I serves as a microphone for the pick-up of sound energy within the class-room and transducers i3, Ill and I5 serve as loudspeakers in the homes. This condition prevails continuously until one of the physically handicapped students listening at home is requested by the teacher to participate in the classroom discussion, or desires to ask a question. Under these circumstances, the student presses a switch 34, 35 or 3.55 at the corresponding home unit and displaces it from the normal Listen to the Talk position.

Actuation of one of the switches 34, 35 or 36 reverses the energy flow relation over the corresponding leased telephone line and, in addition, reverses the eiective operation of transducer II at the school and the transducer connected to the home unit whereat the switch was actuated. Thus, should the student at home unit 3| move switch 3 to the Talk position, transducer I3 operates as a microphone and transducer II operates as a loudspeaker and thercircuits are altered whereby a student speaking into microphone I3 will plainly be heard by the teacher and class listening to the output of loudspeaker II and by all students at the remaining home units such as 32 and 33.

In this manner, it is evident that physically handicapped students confined to their homes may partake actively in discussion and proceedings of a classroom, and that all discussion is heard by all parties to the electrical communication system illustrated. It will be noted that if it is desired to transmit the, discussion occurring in the classroom in which transducer l2 is located, then transducer Il need only be disconnected from the school unit I6 and transducer I2 connected thereto to place the signal output of transducer l2 on the leased telephone line. Y

Having described the broad aspects of the present intercommunication system, reference is now made to the schematic circuit diagram of Figure 2 which illustrates the electrical components comprising school unit I6 associated with transducer il of Figure 1. In the apparatus shown in Figure 2 connecting electro-mechanical resistor for electron tube 5l.

transducer Il and school unit I6, there is inoutput of amplifier lil, as required. The elec-` trical circuit diagram of Figure 2 illustrates. the components in their normal 'condition wherein transducer H is operative as a microphone in a classroom or the like, and audio signals originating thereat are applied at suitable level to outgoing leased line 22 for energizing remote home units.

rIransducer il is preferably a permanent magnet type dynamic unit which, as is well known, is particularly suited for operation as either microphone or loudspeaker. In Figure 2, the voice coil 42 of the transducer H is connected through wiring within the school building to a contact 43 of a-double-pole, double-throw relay 44, the actuating coil 45 of which is normally cle-energized. When relay coil 55 is energized, switch arms d6 and l'i are actuated thereby to interchange connections for purposes to be described in greater detail below. In the condition illustrated in Figure 2, relay switch arm 46 is engaged with contact 43 and thereby couples the output signal of voice coil 132 of transducer ll to the primary coil of an input transformer 5I of amplier 4|.

Generally speaking, amplifier lll comprises two resistance coupled Voltage amplifier tubes driving a beam power output tube. Amplifier 4I and other circuit components are energized from a power supply connected to a suitable alternating current source at plug 52. Closure of switch 53 energizes a power transformer 54 which, in turn, provides lament current for the amplifier tubes and drives the plates of twin diode rectifier 55. At terminal 55, which is the output of a resistance capacity filter 5l, extending from the rectier cathodes, a positive direct voltage is obtained for amplifier operation.

It will be noted that the center tap of coil 6l is connected to ground through a resistor 62 shunted by a suitable filter capacitor 63. This arrangement provides, at terminal 64, a negative direct power source for use in a manner to be described.

The secondary of input coupling transformer 5l is connected between the control grid of amplifier tube 65 and ground through a grid biasing circuit t6. Triode electron tube 6l is energized by the signal output appearing at the plate of amplifier 65 and is connected as a conventional resistance capacitance coupled triode amplier. The output of amplifier tube 6l is utilized to drive the control grid of beam power tube 'I I, the plate of which is returned to positive terminal 56 through the primary winding of the signal output transformer 12. The secondary of output trans- 6 former 12 is connected between ground at one end thereof and switch arm 1l of relay 44. In addition, the secondary of output transformer 12 is utilized to provide an inverse feedback potential which is introduced into the cathode circuit of electron tube 6l. Resistors 13, 14 act as a voltage divider to limit the amount of feedback, and also act as an uri-bypassed cathode bias As illustrated, relay contact arm All is engaged with contact 'i5 when the relay lll is de-energized and accordingly a circuit is made from the secondary of amplifier output transformer l2 through contact 15 and through resistor l5 to the primary winding 'H of a line matching transformer I8. The primary l? of .transformer i8 is shunted by a resistor 8| which-resistor, in series with resistor 16, forms a Voltage divider for the output potential of transformer l2. Thus, the signal applied to line transformer liis limited to that fraction of the output of beam power tube-'ll which is required to suitably energize leased line 2'2 atits predetermined operating level.

The secondary of transformer 18 is split into windings 82 and 83, which windings are shunted by line matching resistors 84 and 85, respectively. The secondary windings 82 and 83 are, however, coupled by means of series capacitor 86, the impedance of which is relatively low when considered at the voice frequency band of transmission. Thus, for voice frequencies, windings 82 and 83 are effectively joined and, as illustrated, are directly coupled by means of lines Sil and 92 to leased line 22 as shown in the drawing.

Thus, even without further description of the elements illustrated in Figure 2, it is apparent that with relay 44 in a de-energized state, as shown, the transducer H may act as a microphone to convert classroom sound into electrical signals appearing in the output of the transducer voice coil 132. These signals are coupled through contact d3 and relay arm 46 to input transformer 5| of amplifier 4I. The amplied output signals appearing at output transformer l2 are coupled through relay arm 41 and contact point l5 to primary coil 11 of line transformer 'i8 through a resistive voltage dividing network comprising resistors 15 and 8| in order that the signal transferred through line transformer T8 be of a level suitable for operation of a leased telephone line without cross-talk or other undesirable interference with adjacent channels. Secondary coils 82 and 83 of transformer 'i8 are series connected by means of capacitor 8S for voice frequency continuity. However, capacitor 85 precludes the flow of direct current between the secondary coils.

One end of coil i5 of relay 44 is connected by lead Se. to the negative potential terminal B4 of the rectifier power source. The opposite end of relay coil :l5 is connected by lead 95 to one end of capacitor 86. The opposite end of capacitor 88 is connected by means of lead 95 to the cathode of beam power tube l! in amplifier (il. By virtue of cathode bias resistor 91 and its associated bypass capacitor 98, the cathode of beam tube 'Il is at an appropriate positive potential. The use is made of the cathode bias circuit as a positive potential source simply because of its ready availability.

It will now become apparent that if a direct current shunt is provided across leased line 22 in some manner and at any point along its length as it extends from the circuit illustrated in Figurer 2, that a direct current path will be provided as follows: from negative potential point 64 through lead Sli, through reiay coil t5, through lead S to transformer secondary 82, through winding 82 to line SI and across the direct current shunt of line 22 (as suggested above) through lead 92 and attached secondary winding 83, and through lead 95 to thepositive potential point at the cathode of beam power tube i I. In other words, if a direct current path is provided between wires of leased line 22, a complete direct current circuit is provided which, in efect, ley-passes capacitor 86 and extends from the positive potentialY point at the cathode of beam tube II to the negative potential point 84 of the power supply, through the Vrelay coil 45, thereby energizing relay 4.

Energization of the relay ifi will disengage the Y relay contact arms and iii from contacts 43 and i5 respectively, and bring the arins into electrical engagement Awith contacts IEIv and H52, respeci ondaries 32 and 23, induce a signal in primary 'ii which in turn is impressed upon the input of amplifier 4I. Resistor F23, and the primary of input transformer 5I shunted by resistor |53 form a voltage divider in this circuit which reduces the incoming signal to a level suitable for application to the input of ampliiier @I without causing overloading or distortion thereof. The energization of relay Liri, by also raising contact arm Iii into engagement with contact 122, conneets the secondary of output transformer 'i2 through the relay arm l to the voice coil d2 of the electrical transducer i i. In summary, therefore, when relay li is energized, audio signals appearing upon leased line 22 will be impressed upon the input of ampliiier 5i, and the output thereof wili be seen to drive transducer ii as a loudspeaker for the reproduction of these audio signals. The net amplification of the circuit from leased line 22 to transducer I i is lowenough to prevent hearing cross-talk from adjacent telephone lines, unless such cross-talk is of nigh enough magnitude to be heard by a standard telephone instrument connected to line 22.

Contact Ii2 of the relay it is connected to the voice coil i2 of the transducer II through a volume level switch IM. In the position shown, the volume level switch Ili has no effect upon the reproduction of the output signal as it passes from contact H32 through voice coil 42. However, when the volume switch is thrown into its opposite position, a step-down transformer HB5 is introduced between the contact m2 and the voice coil (i2. This provides a iixedreduction of volume which is suitable for operation of loudspeaker II at a reduced energy level. It will, of course, be understood that other suitable volume control methods may be employed.

To further understanding of the over-all intercommunication system reference is now made to Figure 3, which illustrates a schematic circuit diagram of the home unit used by the student in listening and partakingin classroom discussions originating at the transducer I I of Figure 2. As-

sum'ing the home' unit of Figure 3 to be that designated as Si in Figure 1, the incoming leased line is correspondingly designated as 24. The transducer and the Listen-Talk switch 32 are also reierenced as in Figure 1.

Although the central exchange supplying the leased line is shown neither in Figure 2 nor Figure I3, it will be understood that the line 22 of Figure 2 connects directly with the line 2 in Figure 3 insofar as signalling is concerned. The Listen-Talk switch Sli of Figure 3 is in the Listen position, corresponding in this system with the de-energized state of relay dit in Figure 2. In other words, transducer II rof Figure 2 is operative as a microphone and transducer It in Figure 3 is operative as a loudspeaker.

With particular reference to Figure 3, the input signal from leased line 25. is applied to split primary windings IIB andV III or" an input line inatchiiig transformer II2. YThe primary wind- "ings Iliand Iiil areshunted by suitable .line

p matching resistors IIS and H2, respectively, and

are seriesl connected for audio frequencies in the band oi operation hy means of a comparatively large capacitor H5. The opposed ends of the capacitor I I5 yare electrically connected by means of leads H6 and II'I to a contact IZB and to switch arm I2I of the Listen-Talk switch 3@ as shown. Arm I2I is not in electrical engagement with contact point I2@ and accordingly, capacitor H is in no way electrically affected thereby. However, closure of contacts IZ and I2! will short-circuit capacitor H5 and through coiis Ii@ and III, provide a direct current path between the wires of leased line 2Q. The results of this circuit change will be considered again hereinbelow.

Secondary winding I22 of the input line transformer I I2 is grounded at one end thereof and at the other end connected through resistor IE5 to contact I23 associated with switch arm i255 of Listen-Talk switch 31%. Winding I22 is shunted by series resistors |25, a volume control variable resistor |27 and Xed resistor I23, With the Listen-Talk switch 34 in the position shown, contact |23 is open-circuited. However, the adjustable output from Volume control i2? is, by

.lead ISI, connected to contact point which,

through engaged switch arm |33 of the Listen- Talk switch 3 is connected to the primary oi input transformer i313 of an ampliiier I35.

`The amplier I35 at the home unit of Figure 3 has been designed for A. C.-D. C. operation. ri'hus, the line plug ISE, when connected to a suitable A. C. or D. C. power line, through switch I3?, energizes the plate of rectier tube The rectified output of this tube is filtered by a resistance capacity filter I il to provide the necessary operating positive direct current potential.

The secondary of the input transformer ili is connected between the grid of voltage amplifier pentode |42 and a common wire Iii-i3, the latter being connected to ground through a suitable capacitor Ulli. Pentode M2 is a conventional high gain audio amplifier tube, the Voutput of which is coupled through capacitor Iii to the control grid of a beam power amplier It, connected in a manner conventional for such tubes. The plate of the beam tube ist is connected to the positive rectified power source through the primary of an output transformer Iii, the secondary of which is connected between ground and switch arm |24. Contact I5I, shown' as engaging switch arm IM, is connected directly to the Voice coil I52 of the transducer I3. As described in connection with transducer of Figure 2, it is preferable that transducer I3 comprise a permanent magnet, dynamic type unit capable of successful microphone or loudspeaker operation.

In viewing the operation of the circuit of Figure 3, with Listen-Talk switch 34 in the position shown, it is apparent that audio signals appearing. on leased line 24 are applied through transformer ||2 and over lead |3| from .the volume control |21 through contact |32 to the switch arm |33 and to input transformer |34 of amplier |35. The amplied output signal is coupled from output transformer |41 through switch arm |24 and contact |5| of the Listen-Talk switch to the voice coil of transducer I3, operating as a loud-speaker. Since, with the relay 44 of Figure 2 in the de-energized state shown, the sound picked up by transducer is converted to audio signals and applied to leased line 22 which in turn is coupled to leased line 24 of Figure 3, it is clear that signals originating in the classroom containing microphone I may be heard at the home containing transducer unit |3.

Operation in this manner, which is the normal state of the circuits, will continue until Listen- Talk switch 34 at the home unit of Figure 3 is pressed to the Talk position. With particular reference to this switch in Figure 8, it will be noted that the yswitch arms |2|, |24 and |33 are mechanicaly ganged and simultaneously operated by the hand lever attached to the switch.

When switch 34 is operated into the Talk position (and a latch may be provided to maintain the switch in this position as long as desired) the three ganged .switch arms |2|, |24 and |33 move upwardly into contact with contacts |29, |23 and |55, respectively. The engagement of switch arm |2| and contact |20 will short-circuit capacitor ||5 and provide a direct current path between windings Il!) and Il and thereby provide a continuous direct current path between the wires of leased line 24.

In the above discussion of Figure 2, it was demonstrated that the provision ofa direct cur,.-

rent path between wiresof leased line'2i (Figure 3) and leased line 22 (Figure 2) would serve to energize direct current responsive relay 44 and reverse the operation of the school unit illustrated in Figure 2; that is, change the operation of transducer from microphone to loudspeaker and reverse the effect of amplifier 4|.

Hence, the operation of Listen-Talk switch 34 at the home unit in Figure 3, reverses completely the mode of operation of the lschool unit shown in Figure 2. In addition to reversing the operation of the school unit, the operation of the home unit is correspondingly reversed to permit reverse transmission of audio signals. Thus, the engagement of switch arm |24 and contact |23 in Figure 3 completes a path from output transformer |41 of amplier |35 to` secondary winding |22 of the line transformer |12. Resistor E26, in series with the parallel combination of the effective input impedance of winding |22 oi transformer ||2, shunted by resistors |25, |2l, |28, acts as a voltage divider for the output of amplifier |35, in order that the signal transferred to line 24 be of a level suitable for operation of a leased telephone line. The engagement of switch-arm |33 with contact arm 555 completes an electrical circuit from voice coil |52 of transducer i3 to input transformer 34 of amplier |35. Transducer I3, operating [as a microphone, converts the speech ot the student impressed upon line matching` transformer ||2.

Thus, when the Listen-Talk-switch in Figure 3 is pressed into the Talk position, sound impinged upon transducer i3 Vwill be heard at transducer ii acting as a loudspeaker. When` this switch is returned to the Listen position;

sound impressed upon transducer ivi will be heard at transducer I3, operating as a loud- The instructor at the school may ask speaker. a question of the student at the terminus of kthe leased line. By simply pressing Listen-Talk switch 34 to the Talk position, the student may reply. Control of reverse operation of the system is wholly within the hands o the studentv whereas `part of the actual control mechanism, namely relay 44, is at the school equip-mentanol responsive to a predetermined (direct current) control signal whenever applied. When the complete system illustrated in Figure 1 is considered in the light of the spe-cinc circuit diagrams of Figures 2 and 3, it will be apparent that a plurality of units, such as shown in Figure 3, may be placed in parallel to accommodate physically handicapped students situated at widespread points. Since all the leased lines are in parallel, it is clear that the operation oi the Listen-Talk switch of any one student will actuate relay 44. However, since all the lines are in parallel, the-audio signal impressed upon the system by one student will be applied to the input circuits of the home units or" all other students in addition to the input circuit of the school unit over leased line 22. Thus, all students on the line will hear the questions and an: swers of all other students.

The basic circuits illustrated in the drawings are known to lend themselves to use in numerous inter-communication systems. Although no attempt has been made to discuss fully all applications to which the circuits illustrated may be placed, it will be apparent that they need not 'be limited to the particular school-student transmission system describedabove.

Thus, in view of the possible modifications'of the above-described system which may be accomplished by those skilled in the. art, it is preferred that the sp-irit and scope oi the present invention be limited solely by the appended claims.

I claim: 1. An electrical intercommunication system comprising electrical transducers at separated points, a two wire transmission line coupling said transducers, electrical switching apparatus for establishing the direction of energy flow vbetween said transducers over said transmission means, said switching apparatus including manually operable means at one end-of said transmission means adjacent one of said transducers and automatically operable means atv the other end of said transmission means adjacent theother'of said transducers, `and means including said two wire transmissionv line-for operating said automatically operablemeans by operation oi ,said manually operalolermeans.` 2. An electrical intercommunication system comprising 'two electrical transducers at 'rated pointscoupled by a two wire transmission -line extending therebetween,v means including.

said transducers for transmitting audio frequency signals over said line electrical switching apparatus for establishing the direction of energy ow between said transducers, said switching apparatus including manually operable means at one end of said transmission means adjacent one of said transducers for applying adi-rect current to said transmission line and automatically operable means at the other end of said transmission means adjacent the other of said transducers responsive to said direct current.

3. An electrical intercommunication system comprising two electrical transducers at separated points, a two Wire transmission line extending between said points and electrically coupling said transducers, means including said transducers for transmitting audio frequency signals over said line manually operable switching means at one end of said transmission means adjacent one of said transducers governing the application of a direct current toy said transmission line, and automatically7 operable switching means responsive to said direct current at the other end of said transmission means adjacent the other of said transducers for governing the reversal of energy flow on said transmission line between said transducers;

4. An electrical intercommunication system comprising two electrical transducers at separated points, a two wire transmission line extending between said points and electrically coupling said transducers, means including said transducers for transmitting audio frequency signals over said line Vmanually operable switching means at one end of said transmission means adjacent one of said ytransducers governing the application of a direct current to said transmission line, and automatically operable switching means responsive to said direct current and at the other end of said transmission means adjacent the other of said transducers for governing the reversal of energy ow on said transmission line between said transducers, said direct current being .applied to said transmission line from a source at the end of said transmission line associated with said automatically operable switching means.`

5. An electrical intercommunication system comprising first and second electrical transducers at first and second separated points respectively, a two wire transmission line extending between said points and electrically coupling said transducers, means including said transducers for transmitting audio frequency signals over said line, rst switching means at one end of said transmission line for governing the reversal of energy flow between said :first transducer and said transmission line and for governing the application of a direct current to said transmission line, second switching means at the other end of said transmission line for governing the reversal of energy ow between said second transducer and said transmission line and responsive to said direct current, and a source of said direct current at the end of said transmission line of said second transducer, said source and said second switching means being connected electrically in series with said transmission line.

l6. In an electrical signalling system a transmission line extending Vbetween rlrst and second separated points, means for interchanging audio frequency energy with said transmission line at said rst and second points comprising audio frequency transformers, each of said transformers having a split winding connected to an end of said transmission line, capacitors electrically coupling the sections of each of said split windings,

a relay and a direct current power source connected at said first end in series relation with said transmission line, and means at said second end for eiectively short circuiting said capacitor at said second end .for energizing said relay from said power source through said transmission line.

'7. In an electricalsignalling system a transmission line extending between rst and second separated points, means for interchanging audio frequency energy with said transmission line at said rst and second points comprising audio irequency transformers, each of said transformers having a split winding connected to an end of said transmission line, capacitors electrically coupling the sections of each of said split windings, a relay and a direct current power source connected at said irst end in series relation with said transmission line, and means at said second end for effectively short circuiting said capacitor at said second end for energizing said relay from said power source through said transmission line, electrical transducers at said nrst and second points coupled to the respective audio frequency transformers, and Vmeans including circuit connections controlled by the operation of the aforesaid relay and said short circuiting means for reversing the direction of energy flow between said transducers.

8. In an electrical intercommunication system, a two wire transmission line extending between rst and secondY separated points, electrical apparatus at said first point comprising an ampliner having input and output circuits, an electrical transducer, an audio frequency transformer having primary and lsplit-secondary windings, a direct current power source and a direct current operated relay, a capacitor kconnecting the sections of said split secondary winding in series relation, the series combination of said sections and said capacitor being connected across said transmission line at said rst point, said relay being connected in series relation with said power source and said transmission line, said transducer being normally connected to said input circuit of said amplier through contacts of, said relay,

' said output circuit of said amplifier being connected to saidv primary winding of said audio transformer through other contacts of said relay, whereby audio frequency electrical signals generated by said transducer are applied to said amplier and the amplied output ofsaid amplifier applied to said transmission line for transmission to said second point, and means at said second point for effectively short circuiting said transmission line for direct current thereby energizing said relay from said powerV source through said transmission line, said relay |when energized being operative effectively to reverse the Vaforesaid. input and output connections of said amplier whereby audio frequencyv signals applied to said transmission line will be received at said rst point, amplified by said amplier and applied to said electrical transducer.

9. In an electrical intercommunication system, a two wire transmission line extending between rst and second separated points, electrical apmission line at said first point, said relay being connected in series relation with said power source and said transmission line, said transducer being normally connected to said input circuit of said amplifier through contacts of said relay, said output circuit of said amplifier being connected to said primary winding of said audio transformer through other contacts of said relay, whereby audio frequency electrical signals generated by said transducer are applied to said arnplier and the amplified output of said amplifier applied to said transmission line for transmission to said second point, means at said second point for effectively short circuiting said transmission line for direct current thereby energizing said relay from said power source through said transmission line, said relay when energized beingr operative effectively to reverse the aforesaid input and output connections of said amplifier whereby audio frequency signals applied to said transmission line will be received at said first point, amplified by said amplifier and applied to said electrical transducer, means at said second point for receiving an audio frequency signal coupled to said transmission line at said first point and amplifying said received signal, and means at said second point for applying an audio frequency signal to said transmission line when said short circuiting means is operative.

10. In an electrical intercommunication system, a two wire transmission line extending between first and second separated points, electrical apparatus at said first point comprising a first amplifier having input and output circuits, a first electrical transducer, `a first audio frequency transformer having primary and split-secondary windings, a direct current power source and a direct current operated relay, a first capacitor conecting the sections of said split secondary winding in series relation, the series combination of said sections and said capacitor being connected across said transmission line at said first point, said relay being connected in series relation `with said power source and said transmission line, said first transducer being normally connected to said input circuit of said first amplifier through contacts of said relay, said output circuit of said first amplifier being connected to said primary winding of said first audio transformer through other contacts of said relay, whereby r v sections and Said second capacitor being connected across said transmission line at said second point, said second transducer being normally connected to said output circuit of said second amplifier through contacts on said switching means. said secondary winding of said second audio frequency transformer being normally connected to said input ,circuit of said second amplifier through other contacts on said switching means, whereby audio frequency signals received over said transmission line from said first point applied to said second amplier and the amplified output thereof applied to said second transducer, said manually operable switching means being arranged to short circuit said second capacitor when operated, and simultaneously to reverse the aforesaid input and output connections of said second amplifier whereby audio frequency signals generated by said second transducer are amplified in said second amplifier and applied to said transmission line through said second audio transformer for transmission to said first point.

11. In an electrical intercommunication system, a two wire transmission line extending between first and second separated points, electrical apparatus at said first point comprising a first amplifier having input and output circuits, a first electrical transducer, a first audio frequency transformer having primary and split-secondary windings, a direct current power source and a direct current operated relay, a first capacitor connecting the sections of said split secondary winding in series relation, the series combination of said sections and said capacitor being connected across said transmission line at said first point, said relay being connected in series relation with said power source and said transmission line, said first transducer being normally connected to said tacts of said relay, said output circuit of said first amplifier being connected to said primary winding of said first audio transformer through other contacts of said relay, whereby audio frequency electrical signals generated by said first transducer are applied to said first amplifier and the amplified output of said first amplifier applied to said transmission line for transmission to said second point: electrical apparatus at said second point comprising a second amplifier having input and output circuits, a second electrical transducer, a second audio frequency transformer having a split-primary winding and a secondary winding, and manually operable switching means,

a second capacitor connecting the sections of said split prim ary winding in series relation, the series combination of these primary sectionsand said second capacitor being connected across said transmission line at said second point, said second transducer being normally connected to said output circuit of said second amplifier through contacts on said switching means, said secondary winding of said second audio frequency transformer being normally connected to said input `circuit of said second amplier through other v whereby audio frequency signals generated by said second transducer are amplified in said second amplifier and applied to said transmission line through said second audio transformer for transmission to said first point, said switching means when operative to short circuit said second capacitor thereby energizing said relay at said first point from said power source through said transmission line, said relay when energized being operative effectively to reverse the aforesaid input and output connections of said first amplifier whereby said audio frequency signals generated by said second transducer will be received at said first point, amplified by said first amplifier and applied to said first electrical transducer.

l2. In an electrical intercommunication system, a two-wire transmission line extending between rst and second separated points, electrical apparatus at said rst point comprising a first amplifier having input and output circuits, a rst electrical transducer, a first audio frequency transformer having primary and split-secondary windings, a direct current power source and a direct current operated relay, a first capacitor connecting the sections of said split secondary winding in series relation, the series .combination of said sections and said capacitor being connected across said transmission line at said first point, said relay being connected in series relation Iwith said power source and said transmission line, said rst transducer being normally connected to said input circuit of said first amplifier through contacts of said relay, said output circuit of said rst amplier being connected to said primary winding of said first audio transformer through other contacts of said relay, whereby audio lfrequency electrical signals generated by said first transducer are applied to said nrst amplifier and the amplified output of said first amplifier applied to said transmission line for transmission to said second point; electrical apparatus at said second point comprisinga second amplifier having input and output circuits, a second electrical transducer, a second audio frequency transformer having a split-primary winding and a secondary winding, and manually operable switching means, a second capacitor con-r ing means, said secondary` winding of said second audio frequency transformer being normally connected to said inputcircuit of -said second amplifier through other contacts on said switching means, whereby audio frequency signals received over said transmission line from said first point applied to said second amplifier and ther ampli- Y fied output thereof applied to said second transducer, said manually operable switching means being arranged to short circuit said second capacitor when operated, and simultaneously to reverse the aforesaid input and output connections of said second amplifier whereby audio frequency signals generated by said second transducer are amplified in said second amplifier and applied to said transmission line through said second audio transformer for transmission to said first point, said switching means when operative to short circuit said second capacitor thereby energizing said reay at said first point from said power source through said transmission line, said relay when energized being operative effectively to reverse the aforesaid input and output connecions of said rst amplifier whereby said audio freqency signals generated by said second transducer will be received at said first point, amplified by said first amplifier and applied to said first electrical transducer, and means at said first and second points for limiting audio frequency currents applied to said transmission line to the predetermined energy level of operation of said transmission line.

13. A communication system adapted for the education of students at points remote from school comprising, electrical apparatus at said school including a transducer for picking up classroom sound and generating corresponding audio frequency signals, means for amplifying said signals, transmission lines extending from said school to said remote points, said amplifying means at said school being coupled to said transmission lines, transducers at said remote points for reproducing said signals, a relay at school apparatus operative when actuated to reverse the direction of energy flow between said classroom transducer and said transmission lines, and means at each of said remote points for actuating said relay and simultaneously reversing the direction of energy flow between said transducer at the remote point from which said relay is actua, ated and the transmission line extending to said remote point, the energy flow relationship Ybetween transducer and transmission line at all remote points other than that from which said relay is actuated remaining unchanged, whereby energy flowing into .said transmission line at said remote point from which said relay is actuated appears at all other remote transducers and at said school transducer.

i4. In an electrical intercommunication system, a two wire transmission line extending between a first point and a central exchange, two wire transmission lines extending between said central exchange and a multiple of remote points, electrical apparatus at said first point comprising a first amplifier having input and output circuits, a first electrical transducer, a first audio frequency transformer having primary and splitsecondary windings, a direct current power source and a direct current operated relay, a rst capacitor connecting the sections of said split secondary winding in series relation, the series combination of said sections and said capacitor being connected across said transmission line at said rst point, said relay being connected in series relation with said power source and said transmission line, said first transducer being normally connected to said input circuit of said first l amplifier through contacts of said relay, said output circuit of said rst amplifier being connected to said primary winding of said first audio transformer through other :contacts of said relay, whereby audio frequency electrical signals generated by said first transducer are applied to said first amplifier and the amplified output of said first amplifier appliedto said transmission line for transmission to yeach of said remote points; electrical apparatus at eachlof said remote points each comprising a second amplifier having input and output circuits, a second electrical transducer, a second audio frequency transformer having a split-primary winding and a secondary winding, and manually operable switching means,

a second capacitor connecting the sections of said split primary winding in series relation, the series combination of these primary sections andjsaid vsecond capacitor being connected across said transmission line at each of said remote points, vin each set of multiple remoteelectrical apparatus the said second transducer being normally connected to said output circuit of said second amplifier through contacts on said switching ,means said secondary winding of said second in each set of remote electrical apparatus the aszaoes amplified output thereof is applied to said second transducer, said manually operable switching means being arranged to short circuit said second capacitor when operated and simultaneously to reverse the aforesaid input and -output connections of said second amplifier whereby audio frequency signals generated by said second transducer are amplified in said second amplifier and applied to said transmission line through said second audio transformer for transmission to said rst point.

v 15. In an electrical intercommunication system, a two wire transmission line extending between a first point and a central exchange, two wire transmission lines extending between said central exchange and a multiple of remote points, electrical apparatus at said first point comprising a first amplier having input and output circuits, a first electrical transducer, a rst audio frequency transformer having primary and splitsecondary windings, a direct current power source and a direct current operated relay, a rst capacitor connecting the sections of said split secondary winding in series relation, the series combination of said sections and said capacitor being connected across said transmission line at said first point, said relay being connected in series relation with said power source and said transmission line, said first transducer being normally connected to said input circuit of said first amplifier through contacts of said relay, said output circuit of said first amplifier being connected to said primary winding of said first audio transformer through other contacts of said relay, whereby audio frequency electrical signals generated by said first transducer are applied to said first amplifier and the amplified output of said first amplifier applied to said transmission line for transmission to each of said remote points; electrical apparatus at each of said remote points each comprising a second amplifier having input and output circuits, a second electrical transducer, a second audio frequency transformer having a split-primary winding and a secondary winding, and manually operable switching means, a second capacitor connecting the sections of said split primary winding in series relation, the series combination of these primary sections and said second capacitor being connected across said transmission line at each of said remote points, in each set of multiple remote electrical apparatus the said second transducer being normally connected to said output circuit of said second amplifier through contacts on said switching means, said secondary winding of said second audio frequency transformer being normally connected to said input circuit of said second ampliiier through other contacts on said switching means, whereby audio frequency signalsA received over said transmission line from said first point are applied to each of said remote amplifiers and in each set of remote electrical apparatus the amplier output thereof is applied to said second transducer, said manually operable switching means being arranged to short circuit said second capacitor when operated and simultaneously to reverse the aforesaid input and output connections of said second amplifier whereby audio frequency signals generated by said second transducer are amplified in said second amplier and applied to said transmission line through said second audio transformer for transmission to said first point, said switching means when operative to short circuit said second capacitor thereby energizing said relay at said first point from said 18 power source throughsaid transmission line, said relay when energized being operative effectively to reverse the aforesaid input and output connections of said first amplifier whereby said audio frequency signals generated by said remote transducer from which said-relay is actuated will be received at said rst point and at all remote points other than that from which said relay is actuated, amplified bysaid iirst amplier and at each of said other remote amplifiers and applied to said first electrical transducer and each of the said remote transducers other than that from which said relay is actuated.

16. In an electrical intercommunication system, a two wire transmission line extending between first and second separated points, electrical apparatus at said first point comprising an amplirier having input and output circuits, an electrical transducer, an audio frequency transformer having primary and secondary windings, a direct current power source and a direct current operated relay, a capacitor connected in series relationship with said secondary winding and said transmission line at said first point, said relay being connected in series relation with said power source and said transmission line, said transducer being normally connected to said input circuit of said amplifier through contacts'of said relay, said output circuit of said amplifier being-connected to said primary winding of said audio transformer through other contacts of said relay, whereby audio frequency electrical signals generated by said transducer are applied to said amplifier and the amplified output of said amplifier applied to said transmission line for transmission to said second point, and means at said second point for effectively short circuiting said transmission line for direct current thereby energizing said relay from said power source through said transmission line, said relay when energized being operative effectively to reverse the aforesaid input and output connections of said amplier whereby audio frequency signals applied to said transmission line will be received at said first point, amplified by said amplifier and applied to said electrical transducer.

17. In a communication system having a first subscriber station and a second subscriber station and a two wire telephone line extending between said subscriber stations, a transformer terminating each end of said two wire telephone line, each of said transformers including a pair of primary windings and a condenser coupling each pair of windings at each terminal, each of said transformers also including secondary windings, a terminal unit at each subscriber station, the terminal unit at said first subscriber station being normally connected through said transformer winding thereat to said two Wire telephone line for transmitting audio frequency signals over said two wire telephone line and said terminal unit at said second subscriber station being normally connected through said transformer winding thereat for receiving said audio frequency signals, a manually operable switch at said second subscriber station for completing an electrical bypass circuit around said coupling condenser, a source of direct current, a relay connected across said coupling condenser at said first subscriber station, said by-pass switch at said second subscriber station completing a direct current energizing circuit from said direct current source over said two wire telephone line for energizing said relay when said switch is closed, circuit connections controlled by said relay wheri energized for selectively connectingA thefterminal' unit at saicl= rst subscriber station for receiving audio frequency signals over said twowire telephone line and-said manually operatedlswitch atsaidsecondsubscriber station connecting said terminal unit aty said second subscriber stationV to transmit audio frequency-signals oversaid two wiretelephone line.

18. In a communication systemvhaving a malin` of saidfirstwindings-at each terminal, each ofv Saidlit'ransformers alsolincluding second windings,

aiterminalunit atl eachstation, the terminal unitA at said-main station being normally connected through said vtransformer winding thereat-to each ofV saidtwo wire lines for transmitting audio frequencysignals over said twov wirelinesand said terminal unit ateachremote station being normally connected through"saidrtransformer winding thereat for. receiving said audio frequency sig-nais, a normally operable -switcl'rV at` eacl'isaid' remote station for completing an electrical`- liy pass` circuit around said coupling condenser thereat, a'source of direct current, afrel'ay'connected across said coupling' condenser at said main station, said by-pass switch at each said' re-I mote stationy completing a direct current energizngfcircuit from said direct current source over said two wire line for energizingy said relay when said switch is closed, circuit connections controlled by said relay when energized? for selectively connecting the terminali uint/fat saidf' main station for receiving audio frequency signals over said tworwire line and said manually operated switch at each said remote station" connecting said terminal unit ther-eat to transmitaudio fre'- quency signals overv said twov wire lines- ALLAN C, BERNSTEIN.

REFERENCES CITED The following references are of record in the' file of this patent:

UNITED STATES PATENTS Number Name Date 1,733,194 Harrisietal. Oct. 29, 1929 2,054,933 Frazier Sept. 22, 1936 2,055,921 Baker ISept. 29, 1936 2,427,496 Feld'scher Sept. 16, 1947 

